-struct Vertical_group_element;
-struct Vertical_align_spanner;
-struct Vertical_align_engraver;
-struct Vertical_align_element;
-struct Voice;
-struct Request_chord;
-struct Voice_group_engravers;
-struct Voice_group_performer;
-struct Voice_list;
-struct Voice_engravers;
-struct Voicegroup;
-struct Walker_engravers;